The invention relates to an automatic focal point sensing device that calculates the degree of focusing by use of an image evaluation function.
It is well-known that the optical image formed by a lens has the maximum amplitude of brightness of the image, or the maximum contrast, when it is in focus. This phenomenon takes place because the intensity of light (power spectrum) at each spatial frequency of the optical image becomes the largest when the image is in focus. A method of sensing the position of the focal point making use of the phenomenon has been considered. Known devices based on such a method are as follows.
In the focal point sensing device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 56-1022, a self-scanning photoelectric device composed of very small photoelectric elements is used. The signal obtained by dividing the difference in photoelectronic output between two very small photoelectric elements by the sum of photoelectric outputs is extracted. The peak value of the signal in a definite scanning period is used as a contrast signal.
In the focus sensing device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 57-207210, nonlinear operation is performed on the outputs of two sets of light-receiving elements provided in front of and behind an estimated focal point plane of the optical system by calculating the difference in output between two preset combinations of light-receiving elements. The nonlinear operation enables the position of the optical system to be sensed. Furthermore, of these sense signals, the difference between two sense signals is computed. From the result, the position of the optical system is determined. In the unfocused region, an analog signal corresponding to the position of the optical system is outputted, whereas in the focused region with a latitude, an in-focus signal of a specific level is outputted.
In the image taking-in device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-78112, charges read with a charge transfer clock from the individual pixels of a CCD image sensor are stored sequentially. On the CCD image sensor, the subject image formed by an image forming optical system is projected. At the same time, resetting is done in a period equal to an integral multiple of the half period of the charge transfer clock and a specific number of adjacent pixels are added. The added charges are sampled. The pixel input range at the time when the subject is taken in from the CCD image sensor is set to the pixel range equal to an integral multiple of the maximum number of pixels that can be added during the reset period. This makes the subject taking-in range constant, improving the image evaluation accuracy.